1. Field of the Invention
The present invention relates to a beam homogenizer to homogenize energy distribution of a beam spot on a surface to be irradiated in a certain region. The present invention also relates to a laser irradiation apparatus to irradiate the beam spot to the surface to be irradiated, and to a method for manufacturing a semiconductor device using the laser irradiation apparatus.
2. Description of the Related Art
In recent years, the research has been conducted widely concerning the technique to perform laser annealing to an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystallinity such as poly-crystal or micro-crystal, which is not single-crystal) formed over an insulating substrate such as a glass substrate. A silicon film is often used as the semiconductor film. It is noted that the laser annealing described here indicates a technique to recrystallize a damaged layer or an amorphous layer formed over a semiconductor substrate or over the semiconductor film on a substrate, a technique to crystallize the amorphous semiconductor film formed over the substrate, and also includes a technique applied to planarize or modify a surface of the semiconductor substrate or the semiconductor film on a substrate.
The glass substrate is inexpensive and is easily processed into a large substrate compared with a quartz substrate which has been often used conventionally. This is the reason why the research has been conducted extensively. A laser beam is commonly employed in crystallization since the glass substrate has low melting point. The laser beam can give high energy only to the non-single crystal semiconductor film without changing the temperature of the substrate that much.
A crystalline silicon film formed by performing the laser annealing has high mobility. Therefore, a thin film transistor (TFT) using this crystalline silicon film is extensively utilized. For example, the TFT is utilized in a liquid crystal electro-optic device, a light-emitting device and the like of monolithic type in which the TFT for a pixel and the TFT for a driver circuit are manufactured on one glass substrate. The crystalline silicon film is formed of a number of crystal grains and thereby it is referred to as a poly-crystalline silicon film or a poly-crystalline semiconductor film.
In the laser annealing process, a pulsed laser beam generated from an excimer laser is shaped into a square spot with several centimeters on a side or into a rectangular spot having a longer side of 10 cm or more at a surface to be irradiated through an optical system and the beam spot is moved relatively to the surface to be irradiated. Since such a method can enhance productivity and is superior industrially, it is preferably employed. It is noted that among the rectangular beam spots, a rectangular beam spot having a particularly high aspect ratio is referred to as a linear beam spot in this specification.
In particular, unlike a punctate beam spot requiring to be scanned from front to back and from side to side, the linear beam spot can provide high productivity since the linear beam spot can be irradiated to the large-sized surface to be irradiated by scanning the linear beam spot only in one direction perpendicular to the direction of the longer width of the linear beam spot. The laser beam is scanned in the direction perpendicular to the direction of the longer width of the linear beam spot since it is the most effective way to scan the laser beam. Because of such high productivity, at present, the laser annealing process is mainly employing the linear beam spot obtained by shaping the beam spot emitted from a pulsed excimer laser through an appropriate optical system.
FIGS. 7A and 7B show an optical system to process the sectional shape of the beam spot into linear on the surface to be irradiated. The optical system shown in FIGS. 7A and 7B is an extremely general optical system. The optical system not only processes the sectional shape of the beam spot into linear, but also homogenizes the energy distribution of the beam spot on the surface to be irradiated at the same time. Usually the optical system to homogenize the energy distribution of the beam spot is called a beam homogenizer. The optical system shown in FIGS. 7A and 7B is also the beam homogenizer.
When XeCl excimer laser (wavelength: 308 nm) is used as a light source, it is preferable that all the optical elements are made of quartz in order to obtain high transmittance. In addition, it is preferable to use the coating which transmittance to the wavelength of the excimer laser becomes 99% or more. When the other excimer lasers having shorter wavelengths are used as the light source, it is possible to obtain high transmittance by using fluorite or MgF2 as the material of the optical system. However, since these materials are crystal, attention must be paid in selecting the cut surface, coating and the like.
First, a side view of FIG. 7A is explained. A laser beam emitted from a laser oscillator 1201 is divided in one direction through cylindrical lens arrays 1202a and 1202b. The direction is referred to as a longitudinal direction in this specification. When a mirror is inserted in the optical system, the longitudinal direction is bent to the direction of the laser beam bent by the mirror. In this structure, the laser beam is divided into four beams. These divided beams are combined into one beam spot once with a cylindrical lens 1204. The beam spots separated again are reflected by a mirror 1207, and then are converged into one beam spot again on a surface to be irradiated 1209 through a doublet cylindrical lens 1208. The doublet cylindrical lens is a lens including two cylindrical lenses. This homogenizes the energy of the linear beam spot in the longitudinal direction and also determines the length in the longitudinal direction.
Next, a top view of FIG. 7B is explained. A laser beam emitted from the laser oscillator 1201 is divided in a direction perpendicular to the longitudinal direction through a cylindrical lens array 1203. The direction perpendicular to the longitudinal direction is referred to as a lateral direction in this specification. When a mirror is inserted in the optical system, the lateral direction is bent to the direction of the beam bent by the mirror. In this structure, the laser beam is divided into seven beams. These divided beams are combined into one beam spot with a cylindrical lens 1205 on the surface to be irradiated 1209. A dotted line shows correct optical path, correct positions of the lens and surface to be irradiated in the case not disposing the mirror 1207. This homogenizes the energy of the linear beam spot in the lateral direction and also determines the length of the lateral direction.
As described above, the cylindrical lens arrays 1202a, 1202b, and 1203 are the lenses to divide the beam spot of the laser beam. The obtained homogeneity of the energy distribution of the linear beam spot depends on the number of the divided beam spots.
Each of the lenses is made of quart in order to correspond with the XeCl excimer laser. In addition, the lenses have their surfaces coated so that the laser beam emitted from the excimer laser transmits through the lenses very much. This makes transmittance of the excimer laser beam become 99% or more per one lens.
The linear beam spot processed through the above structure is irradiated as being overlapped in such a way that the linear beam spot is displaced gradually in the direction of a minor axis of the linear beam spot. With such an irradiation performed, the laser annealing can be performed to the whole surface of the non-single crystal silicon film so as to crystallize it or to enhance its crystallinity.
Some of the beam homogenizers use a reflecting mirror. (For example, patent document: published patent application No. 2001-291681.)
At present, the long linear beam processed through the optical system as above is used to anneal the semiconductor film in a mass-production factory. When the laser annealing is performed, however, there are some problems that a position of a plane having a homogeneous beam spot changes with the frequency of the pulsed excimer laser, that the homogeneity of the beam spot on the surface to be irradiated goes worse due to the cleaning of the window to isolate the gas, which is a laser medium of the excimer laser, from the outside, and so on. Therefore, the laser irradiation apparatus is not yet of high-quality for mass production.